Limiting driver for switch-mode power amplifier

ABSTRACT

A switch-mode RFPA driver includes first and second field-effect transistors (FETs) arranged in a totem-pole-like configuration. The switch-mode RFPA driver operates to generate a switch-mode RFPA drive signal having a generally square-wave-like waveform from an input RF signal having a generally sinusoidal-like waveform. According to one embodiment of the invention, to maximize high-frequency operation and avoid distorting the switch-mode RFPA drive signal, the switch-mode RFPA driver is designed so that its output can be connected directly to the input of the switch-mode RFPA to be driven, i.e., without using or requiring the use of an AC coupling capacitor. The first and second FETs of the switch-mode RFPA driver are designed and configured to limit and control the upper and lower magnitude levels of the switch-mode RFPA drive signal to levels suitable for switching the switch-mode RFPA directly, obviating any need for DC biasing at the input of the switch-mode RFPA.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. patent applicationSer. No. 14/447,452, filed on Jul. 30, 2014, the disclosure of which isincorporated herein by reference in its entirety and for all purposes.

FIELD OF THE INVENTION

The present invention relates to radio frequency power amplifiers(RFPAs). More specifically, the present invention relates to methods andapparatus for driving switch-mode RFPAs.

BACKGROUND OF THE INVENTION

Radio frequency (RF) transmitters are used to transmit RF signals overthe air (or some other transmission medium, such as coaxial cable orother waveguide) to an RF receiver. To compensate for attenuation of theRF signals as they propagate to the receiver, the RF signals areamplified by a radio frequency power amplifier (RFPA), prior to beingtransmitted.

Various emerging and future military and commercial applications requireor will require RFPAs capable of producing very high RF output powers,for example, tens to hundreds of watts, at microwave frequencies. Overthe years, a substantial amount of research has been dedicated toidentifying semiconducting materials that can be used to build RFPAsthat satisfy this dual requirement of high-power and high-frequency. Oneof the most promising semiconducting materials that has been identifiedis gallium nitride (GaN). GaN is a group III/V semiconductor having avery wide bandgap (˜3.4 eV @ 300K) and a very high breakdown field (300V/μm @ 300K). These two attributes are highly desirable since theyafford the ability to manufacture GaN-based transistors with highbreakdown voltages—a necessary requirement for realizing high RF outputpowers at good efficiency. GaN also has a high thermal conductivity(˜2.3 W/cm·K @ 300K), which further facilitates high power operation.

In order for semiconductor-based RFPAs to be capable of operating atmicrowave frequencies, the semiconducting material should also have ahigh carrier mobility. GaN in its bulk form has a moderate carriermobility similar to that observed in silicon. However, the electronmobility can be substantially increased when GaN is used in a highelectron-mobility transistor (HEMT) device topology. FIG. 1 is asimplified cross-sectional drawing of a typical GaN-HEMT 100,highlighting the GaN-HEMT's salient material layers and physicalcharacteristics. The GaN-HEMT 100 includes an AlGaN/GaN heterostructure102 formed on an electrically-insulating or semi-electrically-insulatingsubstrate 104 (e.g., silicon-carbide (SiC), sapphire (Al₂O₃) or silicon(Si)). The different bandgaps of AlGaN and GaN result in formation of aquantum well in the lower-bandgap GaN 106 material, near the AlGaN/GaNinterface. When the AlGaN/GaN heterostructure 102 is formed, chargecarriers (i.e., electrons) from the wider bandgap AlGaN layer 108diffuse into the quantum well in the lower bandgap GaN layer 106,thereby forming a highly-concentrated two-dimensional electron gas(2DEG) 110. Confining the electrons to the 2DEG 110 has the effect ofsubstantially increasing the GaN electron mobility compared to what isobserved in bulk GaN, making the GaN-based HEMT 100 suitable forhigh-frequency operation.

The high-power, high-frequency capability of the GaN-based HEMT has madeit a desirable candidate for building RFPAs that are capable ofoperating at high frequencies and high RF output powers. In recentyears, RFPAs utilizing GaN-based HEMTs have been successfullymanufactured, validating this capability. However, methods and apparatusfor efficiently driving GaN-HEMT-based RFPAs in wideband, high powerapplications are lacking and greatly needed.

BRIEF SUMMARY OF THE INVENTION

Methods and apparatus for driving switch-mode radio frequency poweramplifiers (RFPAs) are disclosed. An exemplary switch-mode RFPA driverincludes first and second field-effect transistors (FETs) arranged in atotem-pole-like configuration. The switch-mode RFPA driver operates togenerate a switch-mode RFPA drive signal having a generallysquare-wave-like waveform from an input RF signal having a generallysinusoidal-like waveform. According to one embodiment of the invention,to maximize high-frequency operation and avoid distorting theswitch-mode RFPA drive signal, the switch-mode RFPA driver is designedso that its output can be connected directly to the input of theswitch-mode RFPA to be driven, i.e., without using or requiring the useof an AC coupling capacitor. The first and second FETs of theswitch-mode RFPA driver are designed and configured to limit and controlthe upper and lower magnitude levels of the switch-mode RFPA drivesignal to levels suitable for switching the switch-mode RFPA directly,obviating any need for DC biasing at the input of the switch-mode RFPA.

In one embodiment of the invention, the RFPA driver is configured to becontrolled by an unbalanced differential driver control circuit, which,operating according to an input RF signal, generates first and secondgate control signals for controlling the gates of the first and secondFETs of the totem pole switch-mode RFPA driver. An optional signalconditioning circuit may also first be employed (i.e., before thedifferential driver control circuit generates the first and second gatecontrol signals) to reduce the low-to-high and high-to-low transitiontimes of the input RF signal and support the production of a switch-modeRFPA drive signal having a square-wave-like waveform.

The RFPA driver apparatus of the present invention is capable ofoperating at high frequencies and high RF output powers, making itparticularly useful for high-power, high-frequency applications, such asmilitary radar and commercial base station systems, for example. In oneembodiment of the invention, the RFPA driver apparatus and associatedswitch-mode RFPA are both integrated in a single gallium nitride based(GaN-based) monolithic microwave integrated circuit (MMIC), with thetransistors of the RFPA driver apparatus and switch-mode RFPA all beinggallium nitride high electron-mobility transistors (GaN-HEMTs). BecauseGaN has a high thermal conductivity and the GaN-based MMIC can be formedon a high-thermal-conductivity substrate (e.g., silicon carbide ordiamond), heat generated by the RFPA can be readily conducted away fromthe active regions of the RFPA driver apparatus and RFPA, therebyprotecting the RFPA driver apparatus and RFPA from being damaged ordestroyed due to excessive heat. The ability to effectively conduct heataway from the RFPA and RFPA driver apparatus also reduces the size andcost of any external cooling system that may be needed. Finally, becausethe RFPA is a switch-mode RFPA, the single GaN-based MMIC can beadvantageously used in battery-operated devices in which battery life isa major concern, such as in handheld and military backpack radios, forexample.

Further features and advantages of the invention, including a detaileddescription of the above-summarized and other exemplary embodiments ofthe invention, will now be described in detail with respect to theaccompanying drawings, in which like reference numbers are used toindicate identical or functionally similar elements.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified cross-sectional drawing of a typical GaN-HEMT,highlighting the GaN-HEMT's salient material layers and physicalcharacteristics;

FIG. 2 is a block level drawing of a radio frequency power amplifier(RFPA) driver apparatus, according to an embodiment of the presentinvention;

FIG. 3 is a schematic diagram of an RFPA driver apparatus, according toan embodiment of the present invention;

FIG. 4 is a schematic drawing of a current-regulating diode, which canbe used to implement the current source in the driver control circuit ofthe RFPA driver apparatus in FIG. 3;

FIGS. 5A and 5B are signal diagrams of first and second driver controlsignals generated by the driver control circuit of the RFPA driverapparatus in FIG. 3; and

FIG. 5C is a signal diagram of the drive signal generated by the drivercircuit of the RFPA driver apparatus in FIG. 3.

DETAILED DESCRIPTION

Referring to FIG. 2, there is shown a block diagram of an exemplaryradio frequency power amplifier (RFPA) driver apparatus 200, accordingto an embodiment of the present invention. The RFPA driver apparatus 200comprises an input signal conditioner 202, a driver control section 204,and a driver 206, which is configured to drive a switch-mode RFPA 208.As explained in the detailed description that follows, the input signalconditioner 202 operates to transform an RF input signal RF_(in) havinga generally sinusoidal-like waveform into a conditioned RF input signalRF_(in)′ having faster edge transitions and a generally square-wave-likewaveform; the driver control section 204 controls operation of thedriver 206, in accordance with the conditioned RF input signal RF_(in)′;and the driver 206 generates a drive signal VG having a generallysquare-wave like waveform and limited low and high magnitude levelssuitable for driving the switch-mode RFPA 208.

FIG. 3 is a schematic diagram of an exemplary RFPA driver apparatus 300designed in accordance with the general block-level representation ofthe RFPA driver apparatus 200 in FIG. 2. The RFPA driver apparatus 300comprises an input signal conditioning circuit 302, a driver controlcircuit 304, and a driver circuit 306, which is configured to drive amain power switch 354 of a switch-mode RFPA 308. In one embodiment ofthe invention the RFPA driver apparatus 300 and switch-mode RFPA 308 areboth formed in a single gallium-nitride-based (GaN-based) monolithicmicrowave integrated circuit (MMIC), with all transistors beingdepletion-mode, n-channel AlGaN/GaN high electron-mobility transistors(referred to simply as “GaN-HEMTs” in the description that follows). Byutilizing GaN-HEMTs the RFPA driver apparatus 300 and switch-mode RFPA308 are able to operate at high frequencies (e.g. several GHz andabove). Further, because GaN has a wide bandgap (˜3.4 eV @ 300K), highbreakdown field (300 V/μm @ 300K), and high thermal conductivity (˜2.3W/cm·K @ 300K), utilizing GaN-based HEMTs in the RFPA driver apparatus300 allows the switch-mode RFPA 308 to generate very high RF outputpowers (tens to hundreds of watts) at high power densities (e.g., >5W/mm). To further facilitate high-power operation the GaN-based RFPAdriver apparatus 300/switch-mode RFPA 308 GaN-based MMIC can bemanufactured on a high-thermal-conductivity heat-sinking substrate, suchas SiC or diamond. The high-thermal-conductivity property of GaN and theheat-sinking substrate allow heat to be readily conducted away from theRFPA 308 and RFPA driver apparatus 300, thereby protecting the RFPA 308and RFPA driver apparatus 300 from being damaged or destroyed due toexcessive heating. The ability to effectively conduct heat away from theRFPA 308 and RFPA driver apparatus 300 also reduces the size and cost ofany external cooling system that may be needed.

In embodiments of the invention in which the RFPA driver apparatus 300is implemented as a GaN-based MMIC using GaN-HEMTs, it and the GaN-HEMTsmay be fabricated using metal-organic chemical vapor deposition (MOCVD),molecular beam epitaxy (MBE), or any other suitable manufacturingprocess. Details of some exemplary MOCVD fabrication processes that maybe used or adapted to fabricate the GaN-based RFPA driver apparatus 300and RFPA 308 may be found in, for example, U.S. Pat. No. 7,230,284 toParikh et al., U.S. Pat. No. 7,364,988 to Harris et al., and Pengelly etal., A Review of GaN on SiC High Electron-Mobility Power Transistors andMMICs, IEEE Transactions on Microwave Theory and Techniques, vol. 60,no. 6, June 2012, all of which are incorporated herein by reference.

In the description of the exemplary embodiments of invention thatfollows, it is assumed that all transistors in the various circuits ofthe RFPA driver apparatus 300 and the RFPA 308 are GaN-based HEMTs. Itshould be emphasized, however, that the invention is not limited toGaN-based-HEMT technology or to developing drive signals for GaN-basedRFPAs. In other words, although the RFPA driver apparatus 300 and RFPA308 are preferably made from GaN-based HEMTs, other types ofdepletion-mode field effect devices, possibly made from othersemiconducting materials, could be alternatively used. Also, although itis preferred that the circuitry of the RFPA driver apparatus 300 (inputsignal conditioning circuit 302, driver control circuit 304, and drivercircuit 306) and RFPA 308 all be formed in a single MMIC, the RFPAdriver apparatus 300 and RFPA 308 could be formed separately, forexample, with the RFPA driver apparatus 300 implemented in a first MMIC,the RFPA 308 implemented in a second MMIC, and both MMICs mounted andconfigured in a hybrid RF power module. In an implementation in whichthe RFPA driver apparatus 300 is implemented separate from the RFPA 308(i.e., both not formed in the same MMIC), the RFPA 308 could then beimplemented, though not necessarily, using a different field-effecttransistor topology than used to implement the RFPA driver apparatus 300(i.e., using a transistor topology other than a HEMT), and/or could bemade from a different type of semiconducting material than that used toform the RFPA driver apparatus 300 (i.e., a semiconductor other thanGaN).

A detailed description of the various circuits making up the RFPA driverapparatus 300 will now be presented, starting with a detaileddescription of the input signal conditioning circuit 302. As shown inFIG. 3, the input signal conditioning circuit 302 comprises a balanceddifferential pair having: a first arm including a first GaN-HEMT 310connected in series with a first load resistor 312; a second armincluding a second GaN-HEMT 314 connected in series with a second loadresistor 316; and a tail resistor 318. The first and second loadresistors 312 and 316 are coupled between a common power supply V_(D)and the drains of their respective GaN-HEMTs 310 and 314. The sources ofthe first and second GaN-HEMTs 310 and 314 are connected together at acommon node 320 (also referred to as the “common source node 320”below). The tail resistor 318 is connected between the common sourcenode 320 and ground or to some other negative voltage source. The firstand second GaN-HEMTs 310 and 314 are substantially identical, both beingdepletion-mode, n-channel GaN-HEMTs and both having a common negativepinch-off voltage V_(P). In fact, all GaN-HEMTs in the exemplary RFPAdriver apparatus 300 described here (i.e., not just the first and secondGaN-HEMTs 310 and 314 have the same (i.e., common) pinch-off voltageV_(P). (In one specific embodiment of the invention, for example, allGaN-HEMTs in the RFPA driver apparatus 300, as well as the main powerswitch (GaN-HEMT 354) of the switch-mode RFPA 308, have a commonpinch-off voltage V_(P)=−3.8V.) It should be emphasized, however, that acommon pinch-off voltage V_(P) is not an absolute requirement, insofaras the claimed invention is concerned.

The gates of the first and second GaN-HEMTs 310 and 314 are biased to DCbias voltages determined by the voltage dividers formed by biasresistors 326, 328, 330 and 332, the power supply voltage V_(D), and thenegative bias voltage V_(NEG). In one embodiment of the invention thebias resistors 326, 328, 330 and 332 have values that set the gate biasvoltage at both gates of the first and second GaN-HEMTs 310 and 314 to avalue close to zero volts. Setting the gate bias voltages to zero voltsallows the RF input signal RF_(in) to be directly coupled to the inputsignal conditioning circuit, via RF coupling resistor 334. Theresistance of the tail resistor 318 is selected so that the DCgate-to-source voltage of both the first and second GaN-HEMT 310 and 314is greater than the pinch-off voltage V_(P) but not so high that anyinput Schottky diodes which may be present at the gates of the first andsecond GaN-HEMTs 310 and 314 are able to clamp the RF input signalRF_(in).

The principal function of the input signal conditioning circuit 302 isto transform the generally sinusoidal-like RF input signal RF_(in) intoa conditioned RF signal RF_(in)′ having faster low-to-high andhigh-to-low transitions. Depending on the application, the RF inputsignal RF_(in) may be modulated or unmodulated. (For example, in anapplication in which the RFPA driver apparatus 300 and RFPA 308 areemployed in a polar modulation transmitter, the RF input signal RF_(in)could be an angle-modulated RF carrier signal.) The input signalconditioning circuit 302 reduces the high-to-low and low-to-hightransition times of the RF input signal RF_(in) by exploiting a firstfeedback path 322 formed from the drain of the first GaN-HEMT 310 to thegate of the second GaN HEMT 314, and a second feedback path 324 formedfrom the drain of the second GaN-HEMT 314 to the gate of the firstGaN-HEMT 310. When the RF input signal RF_(in) is applied to the inputsignal conditioning circuit 302 and begins swinging negative below thebias point (bias point is assumed to be zero volts in the descriptionthat follows), the resistance of the first GaN-HEMT 310 increases,causing the drain voltage of the first GaN-HEMT 310 to be pulled uptoward the supply voltage V_(D). The increasing drain voltage is fed tothe gate of the second GaN-HEMT 314, via the first feedback path 322,thereby reinforcing the gate drive of the second GaN-HEMT 314. As thedrain voltage of the first GaN-HEMT 310 increases, the drain voltage ofthe second GaN-HEMT 314 decreases. The decreasing drain voltage is fedto the gate of the first GaN-HEMT 310, via the second feedback path 324,reducing the gate drive of the first GaN-HEMT 310. Reducing the gatedrive of the first GaN-HEMT 310 while reinforcing the gate drive of thesecond GaN-HEMT 314 in this manner has the effect of accelerating thetransition of the signal produced at the output node 336 (referred to asthe conditioned RF input signal RF_(in)′ below) from high to low (i.e.,compared to if no gate drive reduction and reinforcement were applied).The result is a conditioned RF input signal RF_(in)′ having fasterhigh-to-low transitions than the unconditioned RF input signal RF_(in).On the other hand, for transitions from low to high, when the RF inputsignal RF_(in) begins swinging positive, the resistance of the secondGaN-HEMT 314 increases, causing the drain voltage of the second GaN-HEMT314 to be pulled up toward the supply voltage V_(D). The increasingdrain voltage is fed to the gate of the first GaN-HEMT 310, via thesecond feedback path 324 to reinforce the gate drive of the firstGaN-HEMT 310. As the drain voltage of the second GaN-HEMT 314 increases,the drain voltage of the first GaN-HEMT 310 decreases. The decreasingdrain voltage is fed to the gate of the second GaN-HEMT 314, via thefirst feedback path 322, thereby reducing the gate drive of the secondGaN-HEMT 314. Reducing the gate drive of the second GaN-HEMT 314 whilereinforcing the gate drive of the first GaN-HEMT 310 in this manner hasthe effect of accelerating the low-to-high transition of the conditionedRF input signal RF_(in)′ (i.e., compared to if no gate drive reductionand reinforcement were applied), resulting in a conditioned RF inputsignal RF_(in)′ having faster low-to-high transitions than theunconditioned RF input signal RF_(in).

The conditioned RF input signal RF_(in)′ produced at the output node 336of the input signal conditioning circuit 302 is applied to the input ofthe driver control circuit 304. (Note that in the exemplary RFPA driverapparatus 300 depicted in FIG. 3, the driver control circuit 304 isdriven single-endedly. In other embodiments of the invention it isdriven differentially using the RF drain outputs of the first and secondGaN HEMTs 310 and 314 as the differential drive signals.) The drivercontrol circuit 304 comprises an unbalanced differential pair having: afirst arm including a first GaN-HEMT 338 connected in series with afirst load resistor 340; a second arm including a second GaN-HEMT 342connected in series with a second load resistor 344; and a tail currentsource 346. A supply end of the first load resistor 340 in the first armis connected to a first power supply producing a voltage V₁ (which inone embodiment of the invention is the same voltage V_(DRV) applied tothe driver circuit 306, i.e., V₁=V_(DRV)), and the supply end of thesecond resistor 344 in the second arm is connected to a second powersupply voltage V₂ (which in one embodiment of the invention is the sameas the voltage V₄ applied to the source of the driver transistor 352 inthe driver circuit 308, i.e., V₂=V₄). The gates of the first and secondGaN-HEMTs 338 and 342 are both biased to a gate bias voltage V_(BIAS)having a value higher than the pinch-off voltage V_(P) but not so highthat any Schottky diodes which may be present at the gates of the firstand second GaN-HEMTs 338 and 348 are able to clamp the conditioned RFinput signal RF_(in)′. Biasing the gates of the first and secondGaN-HEMTs 338 and 342 above pinch-off, i.e., V_(BIAS)>V₃+V_(P), alsoensures that a current path is always available for the tail currentsource 346.

The tail current source 346 is connected between the common source node348 and a negative power supply voltage V_(NRAIL). In one embodiment ofthe invention, the tail current source 346 is implemented using acurrent-regulating (CR) diode 400. As illustrated in FIG. 4, the CRdiode 400 comprises a GaN-HEMT 402 and a current-setting resistor 404connected between the source and gate of the GaN-HEMT 402. The CR diode400 produces a constant current I_(SET) of a value determined by theresistance of the current-setting resistor 404. The voltage dropped bythe current-setting resistor 404 (I_(SET)×R_(SET)) is fed back acrossthe gate-source terminals of the GaN-HEMT 402. The fed-back voltage isnegative from gate to source (i.e., (V_(GS)=−(I_(SET)×R_(SET))).Accordingly, any propensity of the CR diode current to deviate fromI_(SET) is opposed by the negative feedback. For example, if the supplyvoltage V_(CRD) was to increase in an attempt to increase the draincurrent through the GaN-HEMT 402, the voltage drop across thecurrent-setting resistor 404 would increase and since the fed-backvoltage is negatively applied across the gate-source terminals of theGaN-HEMT 402 the GaN-HEMT 402 would oppose the increase in current,forcing the drain current back down to I_(SET). So long as the supplyvoltage V_(CRD) is not reduced below a minimum voltage V_(min), the CRdiode 400 is effective at regulating the constant current I_(SET) inthis manner, despite any variation in the supply voltage V_(CRD).

The primary purpose of the driver control circuit 304 is to generatefirst and second gate control signals VG+ and VG− for controllingoperation of the driver circuit 306. When the conditioned RF inputsignal RF_(in)′ increases higher than the gate bias voltage V_(BIAS),the resistance of the first GaN-HEMT 338 decreases, eventually to avalue that is negligible compared to the resistance of the first loadresistor 340. This results in the drain voltage VG+ of the firstGaN-HEMT 338 being pulled down to the DC voltage V₃ at the common sourcenode 348. (In one embodiment of the invention V₃=−8V.) The first andsecond GaN-HEMT 338 and 342 operate as source followers. Accordingly, asthe drain voltage VG+ of the first GaN-HEMT 338 is being pulled down tovoltage V₃, the voltage at the source of the second GaN-HEMT 342 rises,effectively lowering the gate-to-source voltage applied to the secondGaN-HEMT 342 and causing the second GaN-HEMT 342 to turn OFF. With thesecond GaN-HEMT 342 turned OFF, the drain voltage VG− of the secondGaN-HEMT 342 is pulled up to the supply voltage V₂. (V₂=−4V in oneembodiment of the invention.) Subsequently, when the conditioned RFinput signal RF_(in)′ transitions to a value below the bias voltageV_(BIAS), the resistance of the first GaN-HEMT 338 increases, resultingin the drain voltage VG+ of the first GaN-HEMT 238 being pulled up tothe supply voltage V₁. (In one embodiment of the invention, V₁=V_(DRV)and −2≦V_(DRV)≦0V.) Since the first and second GaN-HEMTs 338 and 342also operate as source followers, as the drain voltage VG+ of the firstGaN-HEMT 338 is being pulled up to the supply voltage V₁, the voltage atthe source of the second GaN-HEMT 342 is lowered, effectively increasingthe gate-to-source voltage applied the second GaN-HEMT 342 and causingthe second GaN-HEMT 342 to turn ON. With the second GaN-HEMT 342 turnedON, the drain voltage VG− of the second GaN-HEMT 342 is pulled down tovoltage V₃. FIGS. 5A and 5B are signal diagrams showing the voltagelevels and timing characteristics of the resulting first and second gatecontrol signals VG+ and VG− produced by the driver control circuit 304.As can be seen, the first gate control signal VG+ varies between anupper voltage level V₁ and a lower voltage level V₃, and the second gatecontrol signal VG− various between an upper voltage level V₂ and a lowervoltage level V₃.

The driver circuit 306 comprises a push-pull type structure having firstand second GaN-HEMTs 350 and 352. The first GaN-HEMT 350 is stacked overthe second GaN-HEMT 352 in a totem-pole-like configuration, with thesource of the first GaN-HEMT 350 connected to the drain of the secondGaN-HEMT 352. The drain of the first GaN-HEMT 350 is configured toconnect to a driver power supply V_(DRV), and the source of the secondGaN-HEMT 352 is configured to connect to a source power supply thatproduces a negative supply voltage V₄ less than the GaN-HEMT pinch-offvoltage V_(P). (In one embodiment of the invention, V₄=V₂=−4V.) The gateof the first GaN-HEMT 350 is configured to receive the first gatecontrol signal VG+ while the second GaN-HEMT 352 is configured toreceive the second gate control signal VG−. As illustrated in FIGS. 5Aand 5B, the first and second gate control signals VG+ and VG− both havegenerally square-wave-like waveforms and are preferably 180 degrees outof phase so that ideally only one of the first and second GaN-HEMTs 350and 352 of the driver circuit 306 conducts at any given time. Permittingonly one of the first and second GaN-HEMTs 350 and 352 to conduct at anygiven time is beneficial since it prevents a waste current path fromforming through the first and second GaN-HEMTs 350 and 352.

The purpose of the driver circuit 306 is to generate a drive signal VGfor the main power switch (GaN-HEMT 354) of the switch-mode RFPA 308.The drive signal VG produced by the driver circuit 306 has a generallysquare-wave-like waveform with fast edge transitions and limited andcontrolled high and low magnitude levels. Preferably, the drive signalVG is directly connected to the input of the switch-mode RFPA 308, i.e.,is not coupled to the input of the switch-mode RFPA 308 via an ACcoupling capacitor. Directly connecting the drive signal VG to the inputof the switch-mode RFPA 308 ensures that the waveform of the drivesignal VG remains substantially square-wave like and is not slowed ordistorted by the presence of an AC coupling capacitor. Directlyconnecting the drive signal VG to the input of the switch-mode RFPA 308also avoids the need for any additional components and/or power suppliesthat would be needed to bias the input of the switch-mode RFPA 308 if ACcoupling was to be used, since biasing can be established and set by thedriver circuit 306 and appropriate selection of the driver circuit 306power supplies.

The drive signal VG switches the main power GaN-HEMT 354 of theswitch-mode RFPA 308 ON and OFF as follows. During times when the firstGaN-HEMT 350 of the driver circuit 306 is ON and the second GaN-HEMT 352is OFF, such as, for example, during time t=t1 (see FIGS. 5A and 5B), avoltage VG+=V₁ is applied to the gate of the first GaN-HEMT 350 while avoltage VG−=V₃ is applied to the gate of the second GaN-HEMT 352. Thevoltage VG+=V₁ has a value sufficient to maintain a conducting channelbetween the drain and source of the first GaN-HEMT 350, and the voltageVG−=V₃ has a value sufficiently negative to pinch off the conductingchannel in the second GaN-HEMT 352. Thus charge flows through theGaN-HEMT 350 into the gate of the main power GaN-HEMT 354 of the RFPA308 to turn the main power GaN-HEMT 354 ON. With the second GaN-HEMT 352OFF and the first GaN-HEMT 350 ON, the gate voltage VG applied to thegate of the main power GaN-HEMT 354 is pulled up to level of the driversupply voltage V_(DRV). Conversely, during times when the first GaN-HEMT350 is OFF and the second GaN-HEMT 352 is ON, such as, for example,during time t=t2, a voltage VG+=V₃ is applied to the gate of the firstGaN-HEMT 350 while a voltage VG−=V₂ is applied to the gate of the secondGaN-HEMT 352. The voltage VG−=V₂ has a value sufficient to maintain aconducting channel between the drain and source in the second GaN-HEMT352, and the voltage VG+=V₃ is sufficiently negative to pinch off theconducting channel in the first GaN-HEMT 350. With the first GaN-HEMT350 OFF and the second GaN-HEMT 352 ON, the gate voltage VG applied tothe gate of the main power GaN-HEMT 354 is pulled down to voltage levelV₄ sufficient to pinch off its conducting channel and turn it OFF

FIG. 5C is a signal diagram showing the voltage levels and timingcharacteristics of the resulting drive signal VG produced by the drivercircuit 306. The drive signal VG has very fast rising and fallingtransitions and its high and low drive levels VG_(H) and VG_(L) arelimited and controlled. Specifically, the low drive level VG_(L) islimited to VG_(L)=V₄ (in one embodiment just below the main powerGaN-HEMT 354 pinch-off voltage V_(P)) and the high drive level VG_(H) islimited to the driver circuit supply voltage, i.e., VG_(H)=V_(DRV). Notethat in one embodiment of the invention, the high drive level VG_(H) isadjustable to one of a plurality of different high drive levels, asindicated by the bracket 502 in FIG. 5C. Since VG_(H)=V_(DRV), thisadjustment can be made by simply varying the driver supply voltageV_(DRV). (In one embodiment of the invention in which the pinch-offvoltage of the main power GaN-HEMT 354 is V_(P)=−3.8V, the high drivelevel VG_(H) is adjustable to one of 0V, −1V or −2V.) Thisvariable-drive capability is beneficial since it allows the high drivelevel VG_(H) to be reduced during times the switch-mode RFPA 308 istransmitting only at low output power levels. Lowering the high drivelevel VG_(H) at low output power levels reduces gate leakage andundesirable amplitude-dependent phase distortion (i.e., AM-PMdistortion), thereby improving the output dynamic range of the RFPA 308.

While various embodiments of the present invention have been described,they have been presented by way of example and not limitation. It willbe apparent to persons skilled in the relevant art that various changesin form and detail may be made to the exemplary embodiments withoutdeparting from the true spirit and scope of the invention. Accordingly,the scope of the invention should not be limited by the specifics of theexemplary embodiments but, instead, should be determined by the appendedclaims, including the full scope of equivalents to which such claims areentitled.

What is claimed is:
 1. A method of driving a high-power depletion modeFET, comprising: raising a gate-source voltage being applied across thegate-source terminals of a first depletion mode field effect transistor(FET) to a value greater than a negative pinch-off voltage of the firstdepletion mode FET and sufficient to switch the first depletion mode FETON; while the gate-source voltage being applied across the gate-sourceterminals of the first depletion mode FET is raised to a value greaterthan a negative pinch-off voltage of the first depletion mode FET andsufficient to switch the first depletion mode FET ON, lowering agate-source voltage across the gate-source terminals of a seconddepletion mode FET to a value more negative than a pinch-off voltage ofthe second depletion mode FET to switch the second depletion mode FETOFF; lowering the gate-source voltage being applied across thegate-source terminals of the first depletion mode FET to a value morenegative than the pinch-off voltage of the first depletion mode FET toswitch the first depletion mode FET OFF; while the gate-source voltagebeing applied across the gate-source terminals of the first depletionmode FET is lowered to a value more negative than the negative pinch-offvoltage of the first depletion mode FET, raising the gate-source voltagebeing applied across the second depletion mode FET to a value greaterthan the negative pinch-off voltage of the second depletion mode FET andsufficient to switch the second depletion mode FET ON; forming agenerally square-wave-like gate drive waveform at a driver outputswitching node common to the first and second depletion mode FETs basedon the switching ON and OFF of the first and second depletion mode FETs;and applying the generally square-wave-like gate drive waveform producedat the driver output switching node directly to a gate of the high-powerdepletion mode FET, without having to level-shift the generallysquare-wave-like gate drive waveform before it is applied to the gate ofthe high-power depletion mode FET.
 2. The method of claim 1, wherein thefirst depletion mode FET has a drain coupled to a first driver supplyvoltage and a source, and the second depletion mode FET has a drain thatis coupled to the source of the first depletion mode FET and a sourcethat is coupled to second driver supply voltage.
 3. The method of claim2, wherein a magnitude of the first driver supply voltage is loweredduring times when the high-power depletion mode FET is producing lowoutput power.